A statistical expert is defending his estimate that hundreds of thousands of Pennsylvania voters lack the photo identification they need to vote under a state law whose constitutionality is on trial.

Bernard Siskin, a Philadelphia consultant hired by the plaintiffs in the case, described his research in detail Tuesday as the trial's first witness.

Siskin found that more than a half-million voters either lacked one of the three forms of photo IDs issued by the Pennsylvania Department of Transportation or possess IDs that expire before the Nov. 5 election.